Hubble's Exciting Universe: Beholding the Birth and Death of Stars
Hubble Space Telescope’s high resolution and wide-wavelength sensitivity gave astronomers key new insights into the lives of stars from birth to death. Hubble probed stellar incubators the vast molecular clouds. The telescope surprisingly uncovered the first evidence that planet formation accompanies star birth. The fireworks that accompany star death, from exotic-looking planetary nebula to titanic stellar explosions, unveiled new mysteries to Hubble.
Stars are the building blocks of the universe. They are assembled into giant star clusters, like towns. In turn, the towns and cities are assembled into continents – the galaxies. Stars are the nuclear fusion-driven furnaces for forging the heavier elements, which in turn become the building blocks for life as we know it. Potential habitats for life, planets, are a common byproduct of star birth. They are the coalesced debris left over from material that fell into a newly forming star. Once a star settles into a calm existence, with planets whirling about it, energy from the star can potentially nurture life if the planet is at a comfortable distance from the star. Therefore, Earth’s past and future fate is intimately tied to the behavior and evolution of our nearest star, the Sun.
Understanding the nature of stars across our galaxy puts our Sun in context with stellar formation, spectral classes, and evolution. It wasn’t until the first half of the 20th century that astrophysicists understood the nuclear energy processes that power the Sun, and understood the nucleosynthesis of the heaver elements as a result of stellar evolution.
Our Sun is 5 billion years old, and so we have no “file footage” of how it was born. But stars elsewhere in the galaxy reveal the birthing process. Hubble’s keen vision allowed astronomers to peer deep into gigantic, turbulent clouds of gas and dust where tens of thousands of stars are bursting to life. The Hubble images reveal a bizarre landscape sculpted by radiation from young, exceptionally bright stars. The observations reveal that star birth is a violent process of intense radiation and shock fronts. The intense, ultraviolet radiation clears out cavities in stellar nurseries and erodes material from giant gas pillars that are incubators for fledgling stars.
One such tempestuous environment is the central region of the Carina Nebula in our Milky Way galaxy. Hubble’s view reveals a fantasy-like landscape of dust and gas that are being sculpted by the action of scorching ultraviolet radiation and outflowing stellar winds of charged particles from a grouping of massive stars. These stars are shredding the surrounding material that is the last vestige of the giant cloud where they were born.
The immense nebula contains at least a dozen brilliant stars that are roughly estimated to be at least 50 to 100 times the mass of our Sun. The most unique inhabitant is the star Eta Carinae, which is in the final stages of its brief and eruptive lifespan.
Some stars in stellar nurseries are born inside dense clouds of cold gas. Hubble has observed several of these natal cocoons. The most famous is the trio of giant gas columns in the Eagle Nebula. Dubbed the “Pillars of Creation,” these stellar nurseries are bathed in the scorching ultraviolet light from a cluster of young, massive stars. Streamers of gas can be seen bleeding off pillars as the intense radiation heats and evaporates it into space. Denser regions of the pillars are shielding material beneath them from the powerful radiation.
A later, infrared view of the pillars transforms them into eerie, wispy silhouettes seen against a background of myriad stars. Infrared light penetrates much of the gas and dust, except for the densest regions of the pillars. Newborn stars can be seen hidden away inside the giant columns.
Hubble also has captured in unprecedented detail energetic jets of glowing gas from young stars. These dynamic jets are a newborn star’s birth announcement to the universe. Hubble’s infrared vision captured one such birth announcement in a dusty, turbulent stellar nursery called the Orion molecular cloud complex.
When stars form within giant clouds of cold molecular hydrogen, some of the surrounding material collapses under gravity to form a rotating, flattened disk encircling the newborn star. The fledgling star feeds on gas that rains down on it from the disk. Some of the superheated material spills away and is shot outward from the star in opposite directions along an uncluttered escape route—the star's rotation axis.
Shock fronts develop along the jets and heat the surrounding gas. The jets collide with the surrounding gas and dust and clear vast spaces. The shock fronts form tangled, knotted clumps of nebulosity and are collectively known as Herbig-Haro (HH) objects. These phenomena are providing clues about the final stages of a star’s birth, offering a peek at how our Sun came into existence 4.5 billion years ago.
Hubble has the power to survey the broad range of stellar types. Our galaxy is predominantly low-mass stars than are cooler than our Sun. Stars like our Sun account for only 10 percent of the galaxy’s population. The most opulent stars, blue giant and supergiants, blaze across the galaxy, but are rare and short-lived. Newborn stars are found in grouping called open clusters. Ancient stars, the galaxy’s first homesteaders, dwell in globular star clusters with populations of 1 million stars each.
Hubble has helped astronomers find the birth certificate of a star that has been around for a long time. The star was thought to be as old as 14.5 billion years, which, at first glance, would make it older than the universe's calculated age of about 13.8 billion years. But earlier estimates from observations dating back to 2000 placed the star as old as 16 billion years. This age range presented a potential dilemma for cosmologists.
The new Hubble age estimates reduce the uncertainty, so that the star's age overlaps with the universe's age—as independently determined by the rate of expansion of space, an analysis of the microwave background from the big bang, and measurements of radioactive decay.
Going Out in a Blaze of Glory
Hubble revealed unprecedented details of the death of Sun-like stars. Ground-based images suggested that many of these objects, called planetary nebulas had simple, spherical shapes. Hubble showed, however, that their shapes are more complex. Some look like pinwheels, others like butterflies, and still others like hourglasses. The images yield insights into the complex hydrodynamics that accompany a star’s shedding of its outer envelope.
Turning its vision to the tattered remains of a massive star’s explosive death, Hubble observations of Supernova 1987A revealed three mysterious rings of material encircling the doomed star. The telescope also spied brightened spots on the middle ring’s inner region, caused by an expanding wave of material from the explosion slamming into it.
Hubble also helped astronomers identify a star that was one million times brighter than the Sun before it exploded as a supernova in 2005. According to current theories of stellar evolution, the star should not have self-destructed so early in its life because it was not mature enough. When it exploded, the doomed star was about 100 times more than our Sun's mass. Pre-explosion pictures from the Hubble archive, taken in 1997, reveal the progenitor star was so bright that it probably belonged to a class of stars called Luminous Blue Variables.
Extremely massive and luminous stars topping 100 solar masses, such as Eta Carinae in our own Milky Way galaxy, are expected to lose their entire hydrogen envelopes prior to their ultimate explosions as supernovas. The observations demonstrate that many details in the evolution and fate of massive stars, such as Luminous Blue Variables, remain a mystery.
Hubble probed the tattered, gaseous remains of supernovas. Peering deep inside the core of a supernova remnant called the Crab Nebula, the telescope showed that the crushed core of the exploded star sends out clock-like pulses of radiation and tsunamis of charged particles embedded in magnetic fields.
The collapsed stellar core, called a neutron star, has about the same mass as the Sun, but it is compressed into an incredibly dense sphere that is only a few miles across. Spinning 30 times a second, the neutron star shoots out detectable beams of energy that make it look like it's pulsating. Hubble's sharp view captures the intricate details of glowing gas that forms a swirling medley of cavities and filaments. Inside this shell is a ghostly blue glow that is radiation given off by electrons spiraling at nearly the speed of light in the powerful magnetic field around the crushed stellar core.
Astronomers used Hubble’s powerful vision and the oldest burned-out stars in our Milky Way galaxy to provide a completely independent reading of our universe’s age. In calculating the age of the cosmos, the researchers did not rely on measurements of the universe’s expansion. The researchers calculated that the dead stars, called white dwarfs, are 12 billion to 13 billion years old.
Hubble has studied many massive stars, including one that astronomers have identified as possibly the most luminous star known. The celestial mammoth, called the Pistol Star, releases up to 10 million times the power of the Sun and is big enough to fill the diameter of Earth's orbit. The star unleashes as much energy in six seconds as our Sun does in one year.
The observations also reveal a giant, bright nebula associated with the star that was created by extremely massive stellar eruptions. The astronomers estimate that when the titanic star was formed 1 million to 3 million years ago, it may have weighed up to 200 times the mass of the Sun before shedding much of its mass in violent eruptions. The star’s formation and life stages will provide important tests for new theories about star birth and evolution.